A drilling fluid is a generic term of various circulating fluids required to satisfy drilling work during drilling. The drilling fluid is an important component of drilling engineering, and has functions of suspending drilling cuttings, cleaning a wellbore, stabilizing a well wall, balancing a formation pressure, transmitting a hydrodynamic force, and cooling a drilling bit and a drilling tool. The success of drilling for oil, gas and geothermal resources largely depends on the property of a drilling fluid.
The rheological property is an important property of a drilling fluid and refers to the property of flow and deformation generated by a drilling fluid under an external stress. The rheological property has a significant effect on drilling cuttings and barite suspending, drilling cuttings carrying, well bottom and wellbore cleaning a mechanical rotation speed and a pressure loss in the circulation system. The filtration property refers to the phenomenon that free water in a drilling fluid filtrates into cracks or pores of rock of a well wall. An increase in filtration volumes of free water into the formation easily causes the instability of the well wall and the damage of an oil gas layer. In addition, if the formed mud cake is too thick, drilling accidents such as drilling sticking, a torque loss, and an increase in a circulating pressure loss may occur.
Currently, domestic and international drilling fluid systems mainly include a water-based drilling fluid and an oil-based drilling fluid. The water-based drilling fluid is a polyphase disperse system, where bentonite is dispersed in water used as a continuous phase and various additives are added to regulate the property of the drilling fluid to meet the demand of drilling. However, during deep drilling, the rheological property of the water-based drilling fluid is unstable and difficult to maintain. In addition, the filtration volumes of the water-based drilling fluid are greatly increased under a high temperature, and water going into a reservoir stratum may damage the reservoir stratum and cause the instability of the well wall, so that the rheological property of the water-based drilling fluid easily gets out of control. In the prior art, modified natural polymers and synthetic polymers are used to control the rheological and filtration properties of the water-based drilling fluid under a high temperature, where the polymers are all line-shaped or branched linear water-soluble polymers. The line-shaped polymers are either thermally degraded under a high temperature, losing the effect of reducing the filtration volumes, or cause too high viscosity of a drilling fluid system in a high temperature, reducing the purifying capacity of the wellbore, affecting the rheological property of the drilling fluid, and increasing the circulating pressure loss during drilling while controlling the filtration properties.
Accordingly, it would be desirable to improve drilling fluids to address these and other drawbacks in the known art.